The digitalization of TDM telecommunication networks up to the subscriber stations has, among other things, the advantage that the subscriber concentrators can be of simpler design.
Normally, four-wire lines are required for the transmission of digital data. That means, however, that the advantage of digitalization of the subscriber stations is negated by an additional outlay caused by the four-wire design of the subscriber lines connecting the subscriber stations to the subscriber concentrator as compared to telecommunication systems in which pulse-amplitude-modulated pulses are transmitted between the subscriber stations and the subscriber concentrator.
In view of the foregoing processing, the data transmission on a two-wire line between the digitally operating subscriber stations and the subscriber concentrator is desirable; the two-wire line in this case is utilized on a four-wire basis through appropriate control of timing hybrid circuits allocated to subscriber stations on the one hand and to the subscriber concentrator on the other by transmitting the data alternately in both directions of transmission over the two-wire line on a time-division basis.
To achieve optimal freedom from distortion during this transmission, that is to say, in order to be able to dispense with an equalizing network where a maximum number of subscriber lines is installed in a local telephone exchange, it has been proposed (West German application No. P 23 46 984.3-31, the equivalent of which is U.S. Ser. No. 506,867 filed Sept. 17, 1974 and now U.S. Pat No. 3,974,339, issued Aug. 10, 1976) to carry out the data transmission on such two-wire subscriber lines at a signalling bit rate less than that on the TDM highways coming in at the subscriber concentrator or going out from there. According to this proposal, the bit signalling rate for transmission on the two-wire lines is selected such that for the transmission in both directions of transmission of the PCM words corresponding to one information sampling, the time interval is nearly the same as a sampling pulse frame as used on the TDM highways. Allowance is made for the transit times on the two-wire line and a tolerance interval is provided for the clock-pulse generators concerned. In this case a signalling bit rate is obtained on the two-wire line of about 256 kilobits. In two-wire lines having a 0.4-mm wire diameter, at least 700-m line lengths can be utilized without the need of pulse regeneration.
In the prior method discussed above, the time interval provided for the message itself is relatively brief as compared to the time intervals still to be taken into consideration, namely, transit time, timing-hybrid switch recovery time, and time for a start-of-word bit and a signalling bit.